1. Field of the Invention
The present invention relates apparatuses for cancelling magnetic fields and, more particularly, to magnetic field shielding of a magnetic resonance imaging apparatus.
2. Description of the Related Art
Magnetic resonance imaging (MRI) is a non-invasive technique of imaging the inner part of an imaging object by detecting electromagnetic waves generated during the precession using the nuclear magnetic resonance (NMR) that is a phenomenon involved with precession of the magnetic spin of an atomic nucleus arising from resonance of the magnetic spin of the nucleus under a strong magnetic field when the magnetic field is applied to the atomic nucleus. The MRI is widely used as a medical diagnostics tool to image the inner part of human body.
Ultra-low field NMR and MRI are promising methods in which a magnetic field established by a main magnet that is a core part of conventional NMR and MRI (hereafter collectively referred to as “MRI”) is divided into two roles such as the detection magnetic field from several microTesla (μT) to tens of μT. The main magnet for conventional MRI is required to uniformly form a high magnetic field of 0.1 Tesla (T) to several Tesla. Therefore, the main magnet is large in volume and high in cost.
A main magnet coil for a conventional MRI apparatus is divided into two parts such as a polarization coil and a pickup coil in an ultra-low field MRI apparatus. A prepolarization magnetic field established by the polarization coil polarizes an object to be imaged (hereinafter referred to as “imaging target”). The pickup coil applies a detection magnetic field to the imaging target. Then, a nuclear magnetic resonance signal generated during relaxation of a magnetic spin of a nuclear of the polarized imaging target is detected while the prepolarization magnetic field is turned off. In such an ultra-low field MRI apparatus, a prepolarization coil has only to establish a strong prepolarization magnetic field although uniformity of the prepolarization coil is reduced. Thus, a nuclear magnetic resonance apparatus has a simple structure and the manufacturing cost of the nuclear magnetic resonance apparatus is significantly reduced.
In addition, since the detection magnetic field does not serve to polarize the imaging target, the intensity of the nuclear magnetic resonance signal may be maintained although the strength of the detection magnetic field is low. In the case of high field MRI, a frequency of a pickup signal corresponding to the Larmor frequency that is in proportion to the magnitude of a magnetic field is several tens of MHz. In the case of a detection magnetic field of tens of μT in ultra-low field MRI, a frequency of a pickup signal is several kHz. In the case of a detection magnetic field of several μT in ultra-low field MRI, a frequency of a pickup signal is hundreds of Hz. For this reason, a phenomenon which cannot be observed using conventional high field MRI may be observed.
Since the magnitude of a magnetic field applied during measurement is low, distortion caused by a metal within or around an imaging target is significantly reduced. Thus, the ultra-low field MRI may be applied without difficulty to a person who wears a metal prosthesis. In addition, the ultra-low field MRI may non-invasively obtain an inner image of a metal can. Furthermore, the ultra-low magnetic field MRI may be applied to the security field besides the medical diagnosis that is a field of conventional MRI. For example, the ultra-low magnetic field may complement or replace conventional X-ray that is used to obtain security images.
Unlike a main magnetic field of high field MRI, a prepolarization magnetic field need not be highly uniform. Accordingly, a prepolarization coil is not limited to a complex shape where a shimming coil is added to a hollow cylinder surrounding an object to be measured (hereinafter referred to as “measurement target”) and may have various shapes such as a thick cake-like shape. Since the magnitude of a magnetic field established by a detection magnetic field coil is low, the detection magnetic field coil is spatially enough to be established. There may be provided an open-type design where a measurement target is disposed below a prepolarization coil.
An ultra-low field MRI apparatus requires a gradient magnetic field to add signal generation position information to a magnetic resonance signal, besides the detection magnetic field for NMR. The detection magnetic field and the gradient magnetic field must be very uniform spatially and constant over time. Thus, the earth's magnetic field and an external magnetic field must be prevented from having an influence on the ultra-low field MRI. In addition, a weak magnetic resonance signal generated by the ultra-low field MRI must be prevented from being contaminated by various external magnetic and electromagnetic noises.
There are two methods for overcoming the above problems. One of the methods is an active shielding method. For example, a triaxial magnetic field sensor and a triaxial Helmholtz coil are mounted around an ultra-low field MRI apparatus. The magnetic field sensor measures an external magnetic field. The Helmholtz coil is supplied with current to cancel the measured magnetic field.
The other method is a passive shielding method. For example, a magnetic shield room is provided around an ultra-low field MRI apparatus. The magnetic shield room is basically made of a plurality of sheet materials having very high magnetic permeability such as mu-metal for blocking external static magnetic fields such as the earth's magnetic field and a low-frequency magnetic field and a metal sheet material having high electric conductivity such as aluminum for blocking a high-frequency magnetic field and an electromagnetic wave.
The active shielding method is advantageous in simple implementation, but its shielding efficiency and property are not good. Moreover, a frequency band of a shielded magnetic field is limited to extremely some parts of a static magnetic field and a low frequency. On the other hand, the passive shielding method using a magnetic shield room is advantageous in excellent shielding efficiency and wide frequency band of a magnetic field. However, the magnetic shield room incurs high cost and occupies a wide space. Due to the advantage and the disadvantage, a low-cost MRI apparatus employs an active shielding method.
However, a strong and intermittent prepolarization magnetic field that is essential to an ultra-low field MRI apparatus polarizes or excites a wall of a magnetic shield room to generate strong eddy current. The polarization of the wall of the magnetic shield room and the eddy current generated on the wall interfere with magnetic resonance of a measurement target to make it difficult for the ultra-low field MRI apparatus to obtain an effective magnetic resonance signal.